Process for producing nb-ti super conducting material

ABSTRACT

A PROCESS FOR PRODUCING A NB-TI SUPER CONDUCTING MATERIAL, WHICH COMPRISES SUBJECTING A SUPER CONDUCTING NB-TI ALLOY HAVING B PHASE AT ROOM TEMPERATURES TO A TWO-STEP AGEING TREATMENT COMPRISING A FIRST AGEING TREATMENT AT A LOWER TEMPERATURE FOR A LONGER PERIOD AND A SECOND AGEING TREATMENT AT A HIGHER TEMPERATURE FOR A SHORTER PERIOD.

3, 1972 TAKUYA SUZUKI ETAL 3,682,719

PROCESS FOR PRODU CING NII'TI SUPER CONDUCTING MATERIAL Filed March 9, 1970 OZS ODF OCDQNZA TEMPERATURE OF HEAT TREATMENT T(C) [NVEN I #15 do Patented Aug. 8, 1972 Us. Cl. 14812.7 12 Claims ABSTRACT OF THE DISCLOSURE A process for producing a Nb-Ti super conducting material, which comprises subjecting a super conducting Nb-Ti alloy having ,9 phase at room temperatures to a two-step ageing treatment comprising a first ageing treatment at a lower temperature for a longer period and a second ageing treatment at a higher temperature for a shorter period.

The present invention relates to a process for producing a super conducting Nb-Ti alloy, more particularly relates to an improvement for enhancing the super conducting characteristics, of such alloys as compared with those obtained by a conventional process. According to the invention the ageing treatment is conducted in two steps: the first step at a lower temperature for a longer period, and the second step at a higher temperature for a shorter period.

Recently, Nb-Ti alloys have been used as a most popular super conducting material, and it is well known that the super conducting characteristics of these alloys remarkably change depending upon heat treatments.

This is attributed to the fact that a Nb-Ti alloy, which has been subjected to a solid solution treatment and then rapidly cooled so as to maintain phase at room temperatures, precipitates a phase or to phase from 3 phase when subjected to an ageing treatment, and these precipitated particles prevent the movement of intruding magnetic flux under the so-called mixed state so that a high critical current density is obtained. This ability of preventing (pinning force) the movement of magnetic flux (pinning force) is considered to depend on the shape, size and amount of the precipitated particles, but it is completely unknown how their shape, size and amount aflFect the super conducting characteristics except that the critical current density is improved by the precipitated particles.

Therefore, in the conventional ageing treatments, a single-step treatment has been employed in which the alloys were maintained at a prescribed temperature for a prescribed period, and up to now an optium ageing treatment condition has been searched for in the single-step ageing treatment process.

Through various and extensive studies on the ageing treatments of Nb-Ti alloys, the present inventors have discovered that more enhanced super conducting characteristics of such alloys than those obtained by a conventional process can be obtained by ageing the alloys first at a relatively low temperature for a longer period and then ageing the alloys at a higher temperature for a shorter period.

The present invention shall be described referring to the attached drawings which is a graph showing the relation between the heat treatment temperature and the critical current density in the present inventive process and a conventional process in a comparative way.

A Nb-Ti alloy was subjected to ageing treatments at various temperatures (T C.) for an optimum period (y, hr.) and the critical current density (Jc) was measured. The results are shown by the curve A in the drawing.

An alloy of same composition as the above alloy was subjected to ageing treatments first at a relatively low temperature (T C.) for a longer period (x hr.) than y hr., and then subjected to ageing treatments at various temperatures (T C.) higher than T C. for y hr. Measurements of the critical current density (Jc) of these treatments are shown by the curve B in the drawings.

Summarizing the above, the conventional ageing treatment produces the curve A: T C. y hr. while the twostep ageing treatment according to the present invention produces the curve B: T C. x hr.+T C. y2hr.

The present invention is based on the above discovery that remarkably improved super conducting characteristics and mechanical properties of Nb-Ti alloys can be obtained as compared with those obtained by the conventional process by ageing the alloys at a relatively low temperature for a longer period as the first treatment and then ageing the alloys at a higher temperature for a shorter period as the second treatment as described above.

Specifically, the present invention is a process for producing Nb-Ti super conducting materials which comprises subjecting a super conducting Nb-Ti alloy containing 30- 70% by weight of Nb to a solid solution treatment to transform the alloy from 0c phase to B phase, cooling the alloy to maintain the ,8 phase at normal temperatures, cold working the alloy, and then subjecting the alloy to a two-step ageing treatment which comprises heat treating the alloy at a temperature between ISO-400 C. for a period between 5-250 hours and heat treating the alloy at a temperature between 350-550 C. but higher than the temperature of the precedent heat treatment for a period between 03-10 hours.

The super conducting Nb-Ti alloys used in the present invention include an alloy contaimng 30-70% by weight of Nb, with the balance being Ti, and an alloy containing 30-70% by weight of Nb, with the balance being Ti and up to 4% by weight of one or more of Mo, V, Zr, Ta, Hf, C, 0 and N The reason for limiting the Nb content to 30-70% by weight in the present invention is that a niobium content outside the above range does not give practically useful super conducting characteristics.

According to the present invention, more remarkable improvement in the super conducting characteristics can be obtained as the niobium content increases or the a stabilizing element increases in an alloy containing 30- 70% weight of Ti. However, so far as the niobium content falls within the above range, even an alloy composition having so high a content of niobium as to render the alloy sluggish to the precipitates can produce better qualities through an appropriate selection of ageing treatment conditions than those obtained by the conventional process.

Specifically, the present invention comprises steps of heat treating a Nb-Ti alloy at a temperature between 800-1000 C. to transform the alloy substantially into ,8 phase, then rapidly cooling the alloy in water, for example, cold working the alloy into a desirable form such as a sheet or wire, subjecting the alloy first to a first-step ageing treatment in which the alloy is treated at an appropriate temperature between ISO-400 C., depending on the alloy composition and the working history, for a long period, normally between 5-250 hours, and then subjecting the alloy to a second-step ageing treatment in which the alloy is treated at a temperature between 350- 550 C. but higher than the temperature of the first-step ageing treatment for a short period normally between 03-10 hours, preferably 0.5-6 hours.

The reason for limiting the temperature in the first-step ageing treatment to 150-400", C. is that no remarkable ageing effect can be expected with a treatment below 150 C. even with a longer treatment time; thus no practical usefulness is obtained, while no remarkable improvement in the super conducting characteristics can be obtained with a treatment above 400 C. as compared with those obtained by the conventional process.

Further, the reason for limiting the temperature in the second-step ageing treatment to 350550 C. is that no remarkable improvement in the super conducting characteristics can be obtained with a treatment below 350 C. as compared with those obtained by the conventional process, while the precipitation of particles of on phase or to phase becomes ditficult with a treatment above 550 C.

The present invention will be better understood from the following examples, but the present invention should not be limited thereto.

EXAMPLE 1 A Nb-65T i alloy was heated at 950 C. for one hour, rapidly cooled, wire drawn with 99.9% reduction of area, heated at 300 C. for 127 hours in vacuum and then heated at 375 C. for one hour. The critical current density at 42 K. of the alloy Was 2.5 10 a./cm. when it was placed in an external magnetic field of 30 kilogauss so that the current passed in a perpendicular direction to the line of magnetic force, and the tensile strength at room temperature was 175 kg./mm.

For comparison, an alloy having the same composition and working history showed a Jc value of 1.8 10 a./cm. at best under the optimum treating conditions (heating at 400 C. for one hour) according to the conventional process, and showed a tensile strength of 160 kg./mm.

EXAMPLE 2 A Nb-35Ti alloy was heated at 950 C. for one hour, cooled rapidly, then clad with copper and cold worked into copper clad wire having a Nb-Ti core of 0.25 m. diameter and an out diameter of 0.33 m. with 99.9% reduction of area. The wire was heated at 250 C. for 250 hours in vacuum, and heated at 400 C. for one hour. The 10 value of the material measured under the same conditions as in Example 1 was 2.15 l a./cm. and the tensile strength was 120 kg./mm.

For comparison, a material having the same composition and working history showed a value of 1.65X l0 a./crn. when treated under optimum conditions (heating at 425 C. for one hour) according to the conventional process, and showed a tensile strength of 110 kg./mm.

EXAMPLE 3 A Nb-50Ti alloy with addition of 1% by weight of Ta, 0.28% by weight of oxygen, 0.12% by weight of nitrogen was heated at 1000 C. for one hour, cooled rapidly and cold worked into wire with 99.5% reduction of area. The wire was heated at 250 C. for 120 hours in vacuum and then heated at 400 C. for one hour. The J e value of the above wire as measured under the same conditions as in Example 1 was 2.3 X 10 a./cm.

For comparison, an alloy having the same composition and working history showed a la value of 1.75 X 10 a./cm. when treated under optimum conditions (heating at 425 C. for one hour) according to a conventional process.

EXAMPLE 4 A Nb-Ti-Zr-Ta alloy containing 30% by weight of titanium, 1.5% by weight of zirconium, 2.5% by weight of tantalum, with the balance being niobium and less than 350 p.p.m. of oxygen was heat treated at 950 C. for one hour, cooled rapidly, and cold worked into wire with 99.8% reduction of area. The alloy wire heat treated at 375 for 200 hours in vacuum and then heat treated at 480 C. for 1.5 hours. The Jc value of this material as measured under same conditions as in Example 1 was 1.2x 10 a./cm. at 75 kilogauss.

For comparison, an alloy having the same compositionand working history showed a Jc value of 0.7 10 a./cm. at best when treated under optimum conditions according to a conventionalprocess.

EXAMPLE 5 A Nb-Ti-O-C alloy containing 30% by weight of titanium, 0.2% by weight of carbon, 1.1% by weight of oxygen, with the balance being niobium was heat treated at 950 C. for one hour, quenched in water, cold worked into wire with 99.5% reduction of area. The wire was then heat treated at 300 C. for hours and further heat treated at 430 C. for 5 hours. The Jc value of this material as measured under same conditions as in Example 1 was 1.5 10 a./cm. at 75 kilogauss. This Jc value is the highest one ever obtained in a high magnetic field with super conducting alloys known up to now.

For comparison, an alloy having the same composition and working history showed a J c value of 1.05 X 10 a./cm. at 75 kilogauss at best when treated under optimum conditions according to a conventional process.

As clearly understood from the above examples, the super conducting characteristics and the mechanical properties of super conducting Nb-Ti alloys can be remarkably improved by an appropriate heat treatment according to the present invention. Another remarkable advantage of the present invention is that a certain desirable Jc value can be obtained with a less working ratio. For example, a working ratio more than 99.95% is required in order to obtain a Jc value of 2.2)(10 a./cm. in a conventional ageing treatment, while according to the present invention only 99.5 working ratio is enough for the same purpose.

Therefore, the production line for working these alloys into wire can be very much simplified by the present invention and thus a considerable practical value is obtained.

As compared with a conventional process, better mechanical properties can be obtained by the present invention and thus the present invention is also advantageous for stabilized properties as magnet colings.

We claim:

1. A process for producing super conducting Nb-Ti material, comprising subjecting a super conducting Nb-Ti alloy containing 3070% by weight of Nb to a solid solution treatment to transform the alloy from a phase to 5 phase, cooling the alloy so that the alloy is maintained in substantial {3 phase at normal temperatures, cold working the alloy, then heat treating the alloy between ISO-400 C. for 5-250 hours, and further heat treating the alloy at a temperature between 350550 C. but higher than the temperature of the preceding heat treatment for 0.3-10 hours.

2. A process for producing Nb-Ti super conducting material according to claim 1 in which the alloy is clad with copper before cold working.

3. A process for producing Nb-Ti super conducting material according to claim 1 in which the super conducting Nb-Ti alloy further contains up to 4% by weight of one or more of Mo, V, Zr, Ta, Hf, C, 0 and N 4. A process for producing a Nb-Ti super conducting material, which comprises the steps of heat treating a Nb-Ti alloy at a temperature between 800-1000 C. to transform the alloy into substantially [3 phase, then rapidly cooling the alloy (cold working the alloy into a desirable form such as a sheet or wire, subjecting the alloy first to a first-step ageing treatment in which the alloy is treated at an appropriate temperature between ISO-400 C., depending on the alloy composition and the working history,

for a long period, and then subjecting the alloy to a second-step ageing treatment in which the alloy is treated at a temperature between 350-550 C. but higher than the temperature of the first-step ageing treatment for a short period.

5. A process for producing Nb-Ti super conducting material according to claim 4 in which the super conducting Nb-Ti alloy contains 30-70% by weight of Nb, with the balance being Ti.

6. A process for producing Nb-Ti super conducting material according to claim 4 in which the super conducting Nb-Ti alloy contains 3070% by weight of Nb with the balance being Ti and up to 4% by weight of one or more of Mo, V, Zr, Ta, Hf, C, and N 7. A process as claimed in claim 4, wherein said rapid cooling is performed in water.

8. A process as claimed in claim 4, wherein said firststep ageing treatment is performed for a period of about between 5-250 hours.

9. A process as claimed in claim 4, wherein said secondstep ageing treatment is performed for a period of 0.3- hours.

10. A process as claimed in claim 4, wherein said second-step ageing treatment is performed for a period of 0.5-6 hours.

11. A process as claimed in claim 4, wherein the alloy is clad with copper before the cold working.

12. A process for producing super conducting Nb-Ti material, comprising subjecting a super conducting Nb-Ti alloy containing 70% by weight of Nb to a solid solution treatment to transform the alloy from 0: phase to 6 phase, cooling the alloy so that the alloy is maintained in substantial 5 phase at normal temperatures, cold working the alloy, then heat treating the alloy between 400 C. for at least 5 hours, and further heat treating the alloy at a temperature of between 350550 C. but higher than the temperature of the preceding heat treatment for 03-10 hours.

References Cited UNITED STATES PATENTS 3,268,373 8/1966 Reynolds 148-32.5 3,472,705 10/ 1969 Gregory 14812.7 3,476,615 11/1969 Fairbanks et al. 148--11.5 F 3,511,720 5/ 1970 Raymond 148--12.7

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 148-13 3 

